Mirror bright silver plating

ABSTRACT

A SILVER CYANIDE PLATING BATH FOR ELECTROPLATING A MIRROR BRIGHT SILVER DEPOSIT IS IMPROVED BY THE ADDITION OF HIGHLY SULFATED CASTOR OIL CONTAINING AT LEAST ABOUT 10% COMBINED SO3.

United States Patent ABSTRACT OF THE DISCLOSURE A silver cyanide plating bath for electroplating a mirror bright silver deposit is improved by the addition of highly sulfated castor oil-containing at least about combined S0 BACKGROUND OF THE INVENTION Present commercial electrolytic silver plating processes most commonly use baths composed of silver cyanide, an alkali metal cyanide, and analkali metal carbonate in a water solution. Thesource of the silvermay be a soloble silver anode which is dissolved in the alkali metal cyanide. The deposits from such solutions are generally required to be bright, ductile, smooth, fine-grained, and relatively hard. Depending on the most desired properties for a given purpose, one or more substances may be added to the bath. The baths-must, of course, have long life, operate at suitable current densities, and preferably they should be clear, water-white solutions so that the operator can observe the parts in the bath I For many purposes the silver deposit must have -a bright finislLwAt one time this could only be achieved by bufiing-or burnishing the non-lustrous deposits formed from the'silver cyanide baths. This procedure is costly, wasteful of metal, and time consuming. Continued efforts were made'through many years to develop a bath from which a suitable mirror bright silver deposit could be formed. A particularly effective bath that has beenpr'oposed is descn'bedin US. Pat. No. 2,735,808, which discloses that silver cyanide plating baths can be improved by the addition of insoluble antimony-polyhydroxy. complex compound as a brightening agent, Such additives produce uniform mirrorbright silver deposits over a wide range of current densities-and they are stable on standing and during the-plating process.

- Although the silver plating baths of the aforesaid patent are a marked improvement over the baths previously known in the'art, efforts have continued to further enhance the brightness of the deposit:

- It is known that wetting agents-can be used to improve the brightness of a deposit. Among the known detergents are sulfated fattyacids'ricinoleic acid and sulfated esters and salts thereof. For example, Turkey red oil, which is a product ofthe reaction of cold "sulfuric acid on castor oil has been used asan additive in plating baths. Castor oil is mainlya glyceride ester of ricinoleic acid. Often these wetting agents have been used in combination with sulfide additives. The addition of the sulfides has not proved satisfactory with or without the sulfated acids, estersand soaps in that they lacked stability. It has further been found that the, addition of the sulfatedcompounds such as sulfated.castc n -oil, produced .a harmful effect. To some extent they enhance the brightness of the finish;

on the other hand they reduce the limiting current density, and the solutions become cloudy more rapidly than when they are not present.

The limiting current density is the current density at which there is a tendency of dullness or burning to occur at the protruding or high current density areas of objects being plated. The objects then must be buffed or polished to bring them up to the desired brightness in these areas. By reducing the current density the process be comes more costly because it takes a longer time for the plating to be completed.

With respect to the cloudiness this is caused by Saling out of the wetting agent when carbonates build up to a certain concentration in the bath. In all cyanide type electroplating solutions carbonates build up and in most baths it has been found that the carbonates at a certain concentration deleteriously affect the electroplating Operation. The baths of aforesaid US. Pat. 2,735,808, have the advantage that as much as oz./gallon of carbonates can build up without materially affecting the operation. Yet when sulfated castor oil such as Turkey red oil Was added, despite the enhanced brightness this additive was not satisfactory in that cloudiness appeared when the It is a further object to produce a silver cyanide plating bath which has long life. It is another object to provide a method of plating articles which have a mirror bright finish through a complete range of flash to heavy deposits. A still further object is to provide a silver plating solution which is stable, produces no deleterious decomposition'products and is operable to produce uniform mirror bright silver plate requiring no further burnishing or polishing. Other objects and advantages of the present invention will be apparent from the description and examples following.

It has now been found that the objects are achieved by adding to the plating solution highly sulfated castor oil containing at least about 10% combined S0 As noted above, sulfated castor oil had been tried and found to be limited in its effectiveness. But it is important to note that the sulfated castor oil and other commercially available substances, such as Turkey red oil, which have as the main active ingredient sulfated castor oil,

' which have been used heretofore as the additive con- 3 THE INVENTION In accordance with this invention a silver cyanide mirror bright plating bath is improved by the addition of highly sulfated castor oil containing at least about 10% S It will be noted that the theoretical maximum of combined S0 in sulfated ricinoleic acid is about 22%. Castor oil is mainly a glyceride ester of ricinoleic acid. The highly sulfated castor oil, in accordance with this invention, contains at least about 10% combined $0 which is considerably greater than the 5-7% combined S0 in the sulfated castor oil used heretofore.

The highly sulfated castor oil is used as an auxiliary brightening agent in silver cyanide baths for producing bright silver deposits in order to further enhance the brightness of the deposit. It has been found that in addition to enhancing the brightness of the silver deposits, the additive has the further benefits of extending the limiting current density and extending the life of the bath with respect to clarity of the bath as compared with plating solutions containing the sulfated castor oils which are not highly sulfated. 1'

Even small amounts of the highly sulfated castor oil "are effective in the plating bath. For example, it may be added to the bath in the amount of about 0.5 to grams per liter of plating solution.

THE PLATING BATHS The silver cyanide plating baths to which the-highly sulfated castor oil is added to enhance the brightness of the deposit contain alkali metal cyanide and alkali metal The mixture is boiled until c p u ont k s .p aceandis .theas iluted o .2 11

ml. with water.

It was noted above that this preferred bath is free of tartrates. It has found that when no tartrates are present the antimony content can be decreased to as low as 0.01

gram per liter (0.0015 ounce per gallon) while still obof about 0.2 g./liter to about 5.0 gJliter.

Typically the preferred'baths contain about 17.8 to 48 grams per liter of silver as potassium silver'cyanide, about 75 to 115 grams per liter of free or uncombined alkali metal cyanide, and about 15 to 150 grams per liter of an alkali metal carbonate (based on ,potassiumas the alkali metal ion), 7.5 to 150 grams per liter of glycerine, 0.3

to5.0 grams per liter of antimony.

In general the process or this inventionfinvolves p;

erating a bath at conventional silver platingtemperatures ranging from about to C., but most conveniently carbonate. Preferably the alkali metal is potassium or sodium, but other alkali metals, cesium, rubidium, and lithium, may also be used.

In a preferred embodiment the bath is free of tartrate and contains in addition a soluble complexpf antimony and a polyhydroxy aliphatic compound as a brightening agent. This bath is disclosed in the aforementioned US. Pat. No. 2,735,808, which gives as suitable examples of the polyhydroxy aliphatic components of the complex glycerine, ethylene glycol, sorbitol, and erythritol. It was at about 25 to 28 C. The pH values for the bath range from about 11.5 to 12.5, the optimum value being about 12.0 to 12.3. The cathodic current densities to be used are generally about 5 to amperes per square footxselection of the most suitable current densities depend, for example, on the temperature, the degree of solution agitation, and the composition of the bath. Higher temperatures and more rapid agitation permit the use of higher current densities. In the course of operation-the antimony may be replenished in the form of a complex or'by anodic' disfound that the straight chain polyhydroxy aliphatic compounds preferably are those wherein the hydroxy groups are attached to consecutive carbon atoms. Also, the preferable compounds are those wherein there are as many hydroxy groups as there are C atoms and also those having from two to six C atoms.

It has been stated above that the antimony is the bath in the form of a soluble complex, e.g. potassiumantimony-erythritol complex. However, the exact stoichiometrical relationship of the elements of the complex has solution from a soluble silver antimony-or antimony anode. The highly sulfated castor oil is lost by simple drag-out and additions are made from time to time.

Using a solution according to this invention, the bath has almost water white clarity so that one can watch the work in process of being plated. Itis possible when electroplating with the above described solutions to purify. by filtration through activated carbonwhile maintaining operation with no appreciable "loss of brightener. Also,

added to I I not been determined, nor whether mixtures of complexes 1 of these elements are present. Neverthless, the presence of the several elements in a soluble complex form is all that is required for the electrolyte to perform its desired function. The relative proportions by weight of the complex are: 1 part antimony, l-6 parts of the straight chain polyhydroxy aliphatic compound and 1-4 parts of an alkali metalhydroxide.

All of theabove mentioned complexes are prepared in a similar manner. For example, to prepare a potassiumantimony-glycerol complex there is added 48 g. of glycerol, 24 g. of potassium hydroxide and 50 ml. water to 10 g. of antimony trioxide. The mixture is then heated to boiling until complete solution takes place, after which the voltime is brought to 200 ml. Such a solution will contain 0.04 g. antimony per ml. The antimony content may be adjusted to almost any concentration, but it has been found convenient to make it up to the above value since for 'every ampere hour used in the electroplating process 1 ml. of the antimony concentrate is required. 7

Also, for example, a sodium-antimony-glycerol complex for use with baths containing sodium salts may be prepared as follows: To 10 grams of antimony trioxide, there are added 48 g. of glycerol, 17 g. of sodium hy-' the solution should be agitated ortheworkto be plated should be moved. Y a The term mirror brightness*is used herein and the following considerations are set forth in order to define I this term as clearly as possible. It ;is well known that if a surface is perfectly plane, mirror-like reflectionis ob-- tained. According to the wave theory'of light, a surface reflects like a plane surface as long as there are no elevations or depressions in it which havedimensions exeeed-' ing a small fraction of-the wavelength of light. For a Q given degree of roughness, it also follows that the regularity of the reflection depends not only on the wavelength of the light but also on the angle of incidence. The smoothness of the surface is thereforebest tested with light falling on it at nearly grazing incidence. This may be seen by examining a plate which is just on the border angle, with no measurable diffuse reflectivityz As has been pointed out, the solution of invention enables the electroplating of silvenonan'object a mirror bright finish, which is 'acceptab le to thetrade as silver v plate. 1f is desired obtain a silver galley plate,"

silver anodes containing various percentages of metals such as antimony, bismuth or tin may be used.

Even when the underlying metal has a surface which is dull but has a smooth finish to begin with, by the use of solutions as described above, the surface becomes more and more bright as the plating proceeds and the final appearance is greatly improved if sufilcient thickness of deposit is applied, whereas the usual result gives a rougher and more matte finish.

Example 1 Two liters of a silver plating solution are prepared containing:

Silver cyanide 55 Potassium cyanide 115 Potassium carbonate 15 Potassium-antimony-glycerol complex 1.5 Glycerol 15 Example 2 This example illustrates the performance of Solution A and B under comparable plating conditions. The operating conditions were as follows:

pH 12.3 Temperature 27 C. Agitation Moderate Using identical conditions with respect to relative positions of the silver anode and the cathode, temperature, and degree of agitation, polished cathodes of brass 1" by 3" were first given a strike in a conventional silver strike bath and then plated, one in Solution A and the other in Solution B. The current density in each case was increased at intervals from 7.5 amperes per sq. ft. to 37.5 amperes per sq. ft. In both cases the deposit obtained was mirror bright up to a current density of 30 amperes per sq. ft. In the case of Solution B on further increasing the current density to 37.5 amperes per sq. ft. the edges of the cathode showed burning and dullness. In the case of Solution A (containing the highly sulfated castor oil) the edges remained mirror bright at 37.5 amperes per sq. ft.; an improvement of more than 20% over the Solution B (containing castor oil of lower sulfation).

Additions of potassium carbonate to Solutions A and B produced the following results: When a concentration of 13 oz./gal. (97.5 g./l.) of potassium carbonate was reached Solution B (containing the sulfated castor oil with 3-5% S0 became cloudy and Solution A (containing the highly sulfate-d castor oil in accordance with this invention) remained crystal clear. Further additions of potassium carbonate up to 25 oz./ gal. (187 g./l.) were made to Solution A and the solution remained perfectly clear.

What is claimed is:

1. An electroplating bath for depositing bright silver deposits comprising a tartrate-free alkaline aqueous solution containing about 17.8 to 48.0' grams per liter of silver as potassium silver cyanide, from about to grams per liter of an alkali metal cyanide, from about 15 to 15 0 grams per liter of an alkali metal carbonate, from 0.3 to 5.0 grams per liter of antimony as a soluble complex of an alkali metal, antimony and a straight chain polyhydroxy aliphatic compound, tfrom 7 .5 to grams per liter of a straight chain polyhydroxy aliphatic compound, and 0.5 to 10.0 grams per liter of a highly sulfated castor oil, said sultated castor oil containing at least about 10% to 16% of combined S0 2. An electroplating bath of claim 1 wherein the sulfated castor oil contains at least about 14% combined 'S03.

3. An electroplating bath according to claim 1, wherein the polyhydroxy aliphatic compound is glycerine.

4. In a process for electrodepositing bright silver, which comprises electrolyzing a tartrate-free alkaline solution containing about 17.8 to 48.0 grams per liter of silver as potassium silver cyanide, from about 75 to 115 grams per liter of an alkali metal cyanide, from about 15 to 150 grams per liter of an alkali metal carbonate, from 0.3 to 5.0 grams per liter of antimony as a soluble complex of an alkali metal, antimony and a straight chain polyhydroxy aliphatic compound to impart brightness to the silver electrodeposit, from 7.5 to 150 grams per liter of a straight chain polyhydroxy aliphatic compound, said complex being formed by heating to boiling an aqueous mixture of 1 part antimony compound, 1-6 parts polyhydroxy aliphatic compound, and 1 4 parts alkali metal hydroxide until complete solution is obtained, the improvement of adding to the bath as an auxiliary brightening agent from 0.5 to 10.0 grams per liter of a highly sulfated castor oil, said sulfated castor oil containing at least about 10% to 16% combined S0 5. The process of claim 4 wherein the bath is operated between about 20 to 40 C. with a cathode current density of 1 to 40 amperes per square foot.

References Cited UNITED STATES PATENTS 2,800,439 7/1957 Fischer et a1. 20443 2,735,808 2/ -6 Greenspan 204-46 2,807,576 9/1957 Kardos 204-46 OTHER REFERENCES Melvin DeGroote et al.; Industrial and Engineering Chemistry, Analytical Edition, vol. 3, pp. 243-244 (1931).

GERALD L. KAPLAN, Primary Examiner US. Cl. X.R. 204-46 

